The inventive triterpenoid cyclase catalyzes the formation of tetrahymanol from squalene by a direct cyclization of squalene (Capsi et al. (1968) J. AM. CHEM. SOC. 90:3563–3564; Abe et al. (1993) CHEM. REV. 93:2189–2206). The triterpenoid cyclase also recognizes oxidosqualene as a substrate (Abe & Rohmer (1994) J. CHEM. SOC. PERKIN TRANS. 1: 783–791). In addition to pentacyclic triterpenoids, the squalene tetrahymanol cyclase also catalyzes the formation of tetracyclic triterpenoids (Abe & Rohmer (1991) J. CHEM. SOC. CHEM. COMMUN. 902–903). Tetrahymanol (or gammaceran-3-ol), which is derived from isoprene, is a member of the isoprenoid class. Isoprenoids play an important role as phytohormones and carotenoids, and as components of chlorophyll, ubiquinone, plant resins, oils, and latex. As steroid hormones, isoprenoids effect important functions in animals. The formation of tetrahymanol can be reprimed in Tetrahymena by adding sterols, such as cholesterol (Conner et al. (1968) J. PROTOZOOL. 15:600–605; Conner et al. (1969) J. BIOL. CHEM. 244:2325–2333).
Isoprenoids are also important components of bacterial and eukaryotic membranes. Similar to hopanoids and sterols (such as cholesterol), pentacyclic triterpenoid has tetrahymanol membrane-stabilizing properties (Conner et al. (1968; 1969); Poralla et al. (1980) FEBS LETT. 113:107–110). By restricting the fluidity of the lipid acid residues of membrane lipids, a condensed (membrane-solidifying) effect is achieved above the phase transition temperature; while below the phase transition temperature, the fluidity of the membrane is increased, thus preventing the optimal close packing of fatty acid residues. In addition, the membrane fluidity depends on the fatty acid composition of the membrane lipids. The fluidity of membranes increases in proportion to the levels of unsaturated fatty acids. With temperature changes, organisms are able to regulate the fluidity of their membranes, for example, via the fatty acid composition. Below the phase transition temperature, isoprenoids and unsaturated fatty acids increase the membrane fluidity via a synergistic effect. The inhibition of the synthesis of the cyclic triterpenoids alters membrane stability. This reduced membrane fluidity can be compensated by an increased proportion of polyunsaturated fatty acids (PUFAs) in the membrane, i.e., the content of PUFAs can be increased by inhibition of the triterpenoid cyclase.
The targeted modification of the composition of the fatty acid spectrum by means of gene technology for the commercial production of special fatty acids or oils is described in Napier et al. (CURR. OPIN. PLANT BIOL. (1999) 2:123–127); Murphy & Piffanelli (SOC. EXP. BIOL. SEMIN. (1998) Ser. 57 (PLANT LIPID BIOSYNTHESIS) 95–130); and FACCIOTTI & KNAUF (In: ADV. PHOTOSYNTH. 6: LIPIDS IN PHOTOSYNTHESIS: STRUCTURE, FUNCTION AND GENETICS. Siegenthaler & Murata (eds.) Kluwer Academic Publishers, Netherlands. (1998) 225–248). Thus, the modification of fatty acid composition can be regulated by altering the genes that code for enzymes which directly participate in the fatty acid synthesis, such as desaturases. However, it has been reported that the level of PUFAs in transgenetic organisms was relatively low (Knutzon & Knauf (1998) SOC. EXP. BIOL. SEMIN. SER. 67:287–304).
The knockout or repriming of the gene that codes for triterpenoid cyclase and the resulting deficiency of tetrahymanol may influence membrane fatty acid composition. However, the modified membrane fluidity can be balanced by the production of unsaturated fatty acids.
Although the triterpenoid cyclase protein from Tetrahymena is known and has been purified (Saar et al. (1991) BIOCHEM. BIOPHYS. ACTA, 1075:93–101), it had not been possible to clone the gene for triterpenoid cyclase from Tetrahymena (dissertation of Michal Perzl (1996) at the Faculty of Biology of Eberhard Karls University Tübingen). In previous studies, the gene sequence of triterpenoid cyclase could not be determined by sequencing the purified protein, PCR with degenerative primers, or hybridization with heterologous probes.
The present invention relates to nucleic acids isolated from Tetrahymena which code for a ciliate-specific triterpenoid cyclase. The inventive nucleotide sequences and the polypeptide sequences derived therefrom demonstrate a surprisingly minimal sequence identity to known isoprenoid cyclases. The invention also relates to the use of nucleic acids for the regulation of triterpenoid cyclase expression in a host organism, as well as the targeted knockout or repriming of the triterpenoid cyclase gene. As a result of the altered expression of the triterpenoid cyclase, it is possible to modify and enrich the levels of multiple unsaturated fatty acids in the host organism.